Image transfer material and method of making it



United States Patent Ofifice 2,727,825 Patented Dec. 20, 1955 MAGETRANSFER MATERIAL AND IVIETHOD OF MAKING IT Harold G. Greig, Princeton,N. J., assignor to Radio Corporation of America, a corporation ofDelaware No Drawing. Application September 30, 1950,

Serial No. 187,827

Claims. (Cl. 106-289) The present invention relates to improvements in aprocess of electrostatic printing and to improved materials for use insuch a process.

One well known process of electrostatic printing comprises the followingsteps: (1) inducing an electrostatic charge on a surface of a platecoated with a film of a photoconductive insulating material, (2)exposing the charged plate to a light image for the purpose of releasingthe charge at every point exposed to the light, (3) contacting the platewith a particulate coloring material, usually in the form of coloredparticles adhering to the surfaces of small glass beads, wherein thecolored particles are detached from the beads and adhere to the plate atevery point where the charge has not been released by the light, and (4)contacting the colored image with a sheet of paper or the like totransfer a copy of the image to the paper.

In the above type of process, many types of colored materials have beentried for developing the latent image. Among these coloring materialshave been: metal bronzing powders, colored gums, hard waxes, and resins,dyed resins, dyed lycopodium powder, powdered sulfur, or talcum powder,where the powders are dyed substantially throughout, also powdered dyes,and carbon dust.

Although all of these may be used with some degree of success, theypossess certain disadvantages. Most of them do not provide adequatedepth of color and some have a melting point higher than the scorchingtemperature of paper so that it is diflicult to fix the transfer printby heating it.

The present invention is directed particularly to an improved type ofmedium for applying color to any latent image formed on anelectrostatically charged plate where the image area is charged eithernegatively or neutral and the remainder of the plate is chargedpositively. The preferred form of improved coloring medium, according tothe present invention, does not have the disadvantages pointed out inconnection with previously used media.-

In general, the improved coloring medium of this invention comprises asubstantially spirit-insoluble powder having good dielectric propertiesand having each particle of the powder coated with a precipitated dye.The powder particles are not dyed throughout; neither is the dye merelysmeared on the particle surface; instead, the dye is precipitated on thesurface.

One object of the present invention is to provide an improved medium forimparting transferable color to any latent image when the latent imageis charged negatively and the area surrounding it is charged positively.

Another object of the invention is to provide an improved process forimparting color to a latent image comprising an uncharged areasurrounded by an area which is electrostatically charged positively.

Another object of the invention is to provide an improved medium fordeveloping a latent image carrying a negative electrostatic charge.

Another object of the invention is'to provide an improved developerpowder for electrostatically charged latent images, which produces anexcellent blue-black color relatively insensitive to heat, acid, alkali,and the common solvents such as water, alcohols, and the hydrocarbons.

Still another object of the invention is to provide an improveddeveloper powder for use in facsimile systems.

These and other objects will be more apparent and the invention will bemore readily understood from the detail description of the inventionwhich follows.

The improved coloring matter of the present invention is prepared bytaking a finely powdered material having good dielectric properties as abase and precipitating a dye onto the surface of the powder. Theprecipitated dye is preferably a reaction product of two spirit-solubledyes which may be mixed to produce a spirit-insoluble product. Thepowder base is preferably one which has a relatively low melting pointbut, less desirably, may be a material of high melting point. Byspirit-soluble is meant soluble in any of the lower alcohols or as thisterm is commonly used in the dyeing industry. By good dielectricproperties is meant the ability to hold an electrostatic charge for atleast about 10 seconds without appreciable leakage.

Example An example of preparation of a preferred embodiment of thematerial according to the invention is as follows: 40 parts by weight ofsulfur in finely divided form, as, for example, sublimed sulfur flowers,is formed into a thin slurry or paste by mixing with 50 parts by weightof methanol. 1.5 parts by weight of spirit-soluble nigrosine dye SSB,Color Index No. 864, is added to the paste and the mixture is stirreduntil the dye is completely dissolved in the alcohol and all of thesulfur is thoroughly dispersed. To this is then added 1 part by weightof a spirit soluble dye that is the condensation product of the dye:

AI HZ OH NOFQN=N I N=N Naso -SO Na soflNa and an organic base to givesolvent solubility, such as triamylamine, or an equivalent amount of anyother of several well known dyes which will react with the nigrosine toprecipitate a black dye. Other suitable dyes are the spirit-soluble fastblack dyes which can be prepared as described in United States Patent2,095,077. For example, one may use the spirit soluble fast black dyemade from the dye:

NH: OH

NaSOa SO3Na which is condensed with an organic base to give solventsolubility according to the process described in U. S. Patent 2,095,077.

Enough of the second dye is added, in any case, to cause completeprecipitation of the nigrosine. The two colors are individually solublein alcoholic spirits, but when a solution of one is added to a solutionof the other, an insoluble black pigment is formed. When they are mixedin the presence of sulfur, as described, the pigment is precipitatedonto the sulfur particles.

The product is isolated by filtering. Traces of excess color can beremoved by washing the filter cake with a small amount of methanol orwith warm water until the filtrate runs clear or" color.

The product is then dried at a temperature below the melting pointofsulfur. it dries toa finely divided black powder. In this form, thepowder can be used, under some conditions, as an electrostatic dustingpowder, but,

. for most purposes, its action can be greatly improved by Conditionsand time of ball milling are important from the standpoint of obtainingoptimum electrostatic properties in the product. If it is desired toutilize the powder as a negative developer; i. e., one which will adhereto the exposed areas of a positively charged plate, one hour of ballmilling is suitable for the size sample used in the example. For largersamples, ball milling should be longer. Times of up to 4 hours have beenused. If the powder is not ground, the image produced is faint andblurred and may have a dirty background. The pure pigment alone, withoutthe sulfur base, gives similar results. As ball milling proceeds, thepowder, when tested, will be found to adhere more strongly to the imageareas and to be repelled better by the charged background of the plate.If ball milling is continued for too long a time, the powder begins toadhere grinding it in a ball mill.

very strongly to the image areas, when tested, making I transfer to asheet of paper, and the cleaning of the plate, more difficult. There isalso a tendency for the powder to lose color strength since the pigmentcoat is slowly removed as ball milling continues, with the coloradhering to the sides of the mill. Time and conditions of grindingdepend upon amount and form of color pigment deposited and size and formof the base particles.

Printing procedure One form of printing procedure that has provedsuccessful with the above described developer powder is the following. Apolished aluminum plate is provided with a surface coating of red,amorphous selenium. This plate is charged in the dark by passing itunder a positive corona discharge at high potential; i. e., 6 to 8 kv.on 0.003 diameter wires placed above the plate. The charged plate isexposed to a light image by any well known technique, such as contactprinting, projection, or kinescope scanning, and the latent image isdeveloped; that is, transferable color is imparted to it, by flowing amixture of the developer powder and glass carrier beads over, theexposed surface. The powder adheres to the selenium, probably bytriboelectric effect, in the areas where the induced charge has beenreleased by the exposure to light. It is repelled by the unexposed areaswhich still retain the induced charge. Thus, a negative of the image isproduced.

Transfer of a copy of the image negative to a sheet of paper, or thelike, is made by placing the sheet on the plate and passing it under thecorona wires with the polarity reversed. The paper is thus given a nega'tive electrostatic charge. The image taken off on the paper in thismanner is then fixed by heating to the fusing temperature of the powder.

Glass heads at various sizes can be used to carry the powder. Beadsranging in diameter from 0.015" to 0.040 have given good results. Anamount of powder which is about 1 to 2% of the'weight of the beads hasbeen found to give optimum results. This varies slightly with bead sizeand type of glass.

Images can be developed on insulating surfaces other thanphotoconductive insulators such as selenium. Varnished or resin coatedmetal plates or glass plates can, for example, be used in a processwhich difiers from that described above in that after a corona-inducedpositive charge has been applied to the surface, this charge is removedlocally to form a latent image by contact with a conductor. Thedeveloper power is then applied to the plate and is retained in theimage areas. The improved developer powder can thus be used to applycolor to any latent image having an electrostatic charge contrastingwith that of the surrounding area, when the image area, itself, either,carries no charge or is negatively charged. The image should notv absorbthe dye or the powder to any extent. Usually, the

color adheres most strongly to the edges of the image at the boundarybetween the areas of contrasting charge. The color can be made to appearover the whole image surface, however.

In preparing the improved developer, concentrated solutions of the dyesshould beused to give best results. This is probably becauseprecipitation of a pigment from a concentrated solution normally favorsformation of larger pigment particles. Either dye can be added to thesulfur slurry first and precipitated by addition of the other color dryor in solution.

Although it is preferred to use a base particle, such as sulfur, havinga relatively low melting point compared with the scorching temperatureof paper, many other materials can be used in finely divided form withsomewhat poorer results. Examples are lycopodium powder, diatomaceousearth, glutamic acid, and l-phenyl-3-methyl- 5-pyrazolone. It isnecessary only that the particle have good dielectric properties andthat it be substantially insoluble in the alcohols used to dissolve thedyes. Where the particle has too high a melting point to be set byfusing on the paper, setting may be accomplished by spraying withlacquer or some other binder.

Any combination of spirit-soluble dyes may be used which will react toprecipitate a color pigment. Nigrosine is preferred as one of the twobut any one of many known spirit-soluble dyes can be mixed with it orthey can be mixed together with no nigrosine present. Others areSolventol Blue, Solveutol Blue 6G, and Solventol lack 2-3164.

T he preferred form of the developerpowder which has been described hasproved to have exceptionally fine properties for making negativeelectrostatic prints in one type of facsimile system. The powderproduces black prints of good contrast. it is easy to prepare, requiringlittle or no classification in order to obtain a narrow range ofparticle size. It is clean to handle and may be removed relatively easyfrom hands and clothing without staining. it is not tacky and has notendency to wot a selenium surface. Thus, it can be cleaned readily fromthe printing plate, a factor of importance in a continuous printingprocess. The pigments also have excellent light fastness. There has thusbeen described an improved developer powder for latent images carryingeithera negative electrostatic charge or no electrostatic charge whenthe surrounding areas are positively charged, and an improved processfor applying transferable color to an electrostatically charged'latentimage. The sameeifect cannot be obtained by merely grinding togetherbase particles and pigment or dye particles. For example, 20 partssulfur and 4 parts carmine were ball milled. This gave a reverse printusing the process described above. That is, the color adhered to allpositively charged areas.

I claim as my invention:

1. An image transfer material comprising a substantiallyspirit-insoluble, inert, dielectric powder having a surface coating of aspirit-insoluble, precipitated organic pigment, said pigment having beenprepared by precipitating spirit-soluble nigrosine with a spirit-solubledye selected from the class consisting of the condensation product ofthe dye NH: OH

condensation product of the dye and an organic base to give solventsolubility upon said powder while said powder is suspended in a spiritmedium.

2. A material according to claim 1 in which said powder is sulfur.

3. A material according to claim 1 in which said surface coating is aprecipitant which is the reaction product of spirit-soluble nigrosineand a spirit-soluble black dye which is the condensation product of thedye I NaSO SO Na SOQNa and an organic base to give solvent solubility.

4. A method of making an image transfer material comprising suspending asubstantially spirit-insoluble inert dielectric powder in a spiritmedium and then precipitating spirit-soluble nigrosine with aspiribsoluble dye selected from the class consisting of the condensationproduct of the dye NHz (3H NaSOa and an organic base to give solventsolubility and the condensation product of the dye ITTH: on 5 N02ON=NN=N SO:4Na

NaSOa and an organic base to give solvent solubility on the sur- 10 faceof said powder while suspended in said spirit medium.

5. A method according to claim 4 in which said powder is sulfur.

References Cited in the file of this patent

1. AN IMAGE TRANSFER MATERIAL COMPRISING A SUBSTANTIALLYSPIRIT-INSOLUBLE, INERT, DIELECTRIC POWDER HAVING A SURFACE COATING OF ASPIRIT-INSOLUBLE, PRECIPITATED ORGANIC PIGMENT, SAID PIGMENT HAVING BEENPREPARED BY PRECIPITATING SPIRIT-SOLUBE NIGROSINE WITH A SPIRIT-SOLUBLEDYE SELECTED FROM THE CLASS CONSISTING OF THE CONDENSATION PRODUCT OFTHE DYE